Optically detected magnetic resonance experiments have been performed for photo-excited [Rh(thpy)2(bpy)]+ doped in a single crystal of [Rh(phpy)2(bpy)]PF6. The results provide unambiguous evidence for a photo-induced triplet state excitation, localized on one of the thienylpyridyl anions chelated to Rh3+. The zero-field splitting parameters of the thienylpyridyl anion in the phosphorescent triplet state were determined to be \D\ = 3445 MHz and \E\ = 865 MHz.